Two proteins, barnase, the extracellular ribonuclease of Bacillus amyloliquefaciens, and barstar, its intracellular inhibitor, are used as a model system for the study of protein folding and protein-protein interactions. Barnase is one of an homologous group of ribonucleases occurring in both prokaryotes and eukaryotes. Recombinant DNA techniques are being applied to the project with three major aims; 1) to facilitate production, 2) to examine the structural and control sequences of the genes and 3) to tailor specifically designed modifications in the sequences to test theories of protein folding. The barnase gene carrying an inactivating transposon and a fragment of the wild type gene overlapping the transposon insertion site have been cloned in E. coli plasmids. Reconstructions demonstrate that the native gene is lethal in E. coli. The entire gene has been sequenced, confirming the amino acid sequence and revealing a signal sequence typical of exported proteins. The crippled gene is transcribed in both E. coli and B. subtilis.